Shannon, bless you for your concern for your friend. As I see it, you need information about your friend, about the disease, and about resources to be able to plan properly on how you can best support her.
Normally you might not want to pry into someone's personal and financial affairs but I think you are going to have to do so here. Have you talked to her family? What are they willing to do to help her? You need to know what is going on here. Are they rejecting her, in denial about her illness and therefore her needs, or is it possible that she has pushed them away?
Has your friend been properly represented in her divorce and the custody proceedings? How will she support herself? Can she still work? Is she eligible for disability?
Have you learned all you can about the disease? We have links to lots of information and you can also send for material from the HDSA. Their URL is www.hdsa.org I recommend Understanding Behavior in Huntington's Disease and the Physician's Guide to the Management of Huntington's Disease.
While you are visiting the HDSA web site, see if you can locate an HDSA chapter near you. Call them and find out what resources are available. Do they have a social worker who can help your friend hook up with local services? Support groups?
What symptoms does she have? Are there any psychological symptoms that may make it difficult for the two of you to share a home? Irritability, inflexibility, compulsiveness? If so, you may need to have an understanding upfront that she will continue to take medication to address these symptoms.
There is a wonderful online community of people dealing with HD. There are mailing lists such as Hunt Dis and HD caregivers, and support clubs such as Yahoo HD Support. We have links to these. I find it very helpful to discuss my concerns and experiences with others.